Hard-core detainees at Guantanamo Bay routinely hurl their waste and vomit at their guards in daily, stomach-turning acts of defiance, a top Pentagon official told lawmakers.

"Splashing," as Marine Gen. John Kelly, the commander of U.S. Southern Command, called the practice in testimony before the Senate Armed Services Committee, is just one of the occupational hazards faced by the men and women guarding some 122 prisoners who remain at the compound. Kelly later elaborat...